Rare earth phosphates, especially orthophosphates represented by the compositional formula: RPO4 wherein R is a rare earth element selected from scandium, yttrium, and lanthanoids of atomic numbers 57 to 71, are known as host crystals for commercial phosphors, as typified by LaPO4:Ce3+, Tb3+ (see “Phosphor Handbook,” Ohm Co., Ltd., 1987, p. 218). So far, the additive element attributable to emission was limited to Ce, Tb, and Eu among other rare earth elements.
Manganese is often used as an activator or dopant to phosphors. The emission of manganese dopant has a peak wavelength almost in the range of 550 to 700 nm which corresponds to yellow to red color. Known materials capable of luminescence of shorter wavelength include calcium fluoride with Mn solid solution, i.e., CaF2:Mn2+ that exhibits a peak wavelength 495 nm, and oxides such as Zn2SiO4: Mn2+ that exhibits a peak wavelength 525 nm and ZnGa2O4:Mn2+ that exhibits a peak wavelength 510 nm (see “Phosphor Handbook,” Ohm Co., Ltd., 1987, pp. 105-108, 231), and Mg2SnO4:Mn that exhibits a peak wavelength 500 nm (see Journal of Luminescence, Elsevier Science, 2002, Vol. 99, pp. 169-173).